In carbon dioxide gas welding, a shielding gas is CO2 and since the electrical gradient plasma is a great, arcing tends to occur from a point with the shortest possible distance, thus such arcing points concentrate in the vicinity of the lower portion of globules. Consequently, current density of the arc column is heightened and arcing results in a firm slender shape. Because of the concentration of arcing points, the arcing force is applied in a direction where departure of globules is blocked, causing a phenomenon where globules are pushed back to the wire upside, therefore, a short circuit transfer of globules occurs in a relatively low-current region, an increase in current does not result in a spray transfer as in inert gas shielded arc welding but merely results in extremely large globules and short circuit transfer and globular transfer exist in a mixed fashion. As a result, arcing stability is lost and spatters frequently occur.
High-speed carbon dioxide gas welding where welding is carried out while increasing the welding rate from the order of 1 m/minute results in high-speed large-current welding, and effects of an electromagnetic pressure caused by electromagnetic force of arcing itself become considerable, influences thereof are also reflected in bead formation, the surface of a pond of weld receives the effects and is thus depressed, whereby a gauging head region where the bottom portion of the pond of weld or the peripheral portion thereof is exposed is formed. Furthermore, a problem exists such that, when effects of arc pressure become strong, namely, when a large current occurs, the gauging region is expanded, and finally, solidification proceeds before the region has been sufficiently buried by the exposed portion, whereby undercuts of humping beads are produced.
In order to solve such problems of high-speed welding, it is generally considered that employment of a buried arc method with a voltage lower than its original welding voltage and a shortened arc length is effective in suppression of occurrence of large-sized spatters and in suppression of undercuts and humping beads.
As in the above, according to the prior art, in high-speed carbon dioxide gas welding at a rate of approximately 1.5 m/minute or more, employment of a buried arc method with a shortened arc length can suppress occurrence of large-sized spatters to some extent, whereas the arc heat source is buried inside the surface of the base metal, resulting in deep penetration. In addition, since the welding rate is high and the welding current is great, the aforementioned gauging region is expanded, solidification progresses at the peripheral portion of the pond of weld, undercuts and humping beads are produced, and, as shown in FIG. 8, the penetration shape of a welded joint part results in a shape (a shape like a mushroom) which is slender and deep at the depths of penetration, is of a convex bead widening at the bead surface portion, and has a narrow part therebetween, therefore, it is possible to obtain a smooth bead penetration surface shape as in MAG welding and a problem concerning welding quality that may remarkably deteriorate notch toughness of the joint part has existed.
Therefore, it is an object of the present invention to solve the aforementioned problems and to provide, in high-speed carbon dioxide gas welding at a rate of approximately 1.5 m/minute or more, an arc welding method which suppresses spattering by employing not a buried arc method with a shortened arc length but an open arc method for traveling of globules with a proper volume of globules and which makes it possible, without producing undercuts or humping beads, to obtain a smooth bead penetration surface shape as in MIG or MAG welding.